The dehydrochlorination of chlorinated hydrocarbons for the preparation of unsaturated compounds is a common synthetic process at industrial level.
For instance, vinylidene chloride is prepared commercially by the dehydrochlorination of 1,1,2-trichloroethane using calcium or sodium hydroxide. The process produces vinylidene chloride in very high yields (approximately 90%) but has the drawback that great amounts of inorganic by-products (e.g. CaCl2 or NaCl) are generated and need to be disposed of or recycled. Catalytic processes producing hydrogen chloride as by-product would therefore be more advantageous.
U.S. Pat. No. 2,361,072 (DU PONT) Oct. 24, 1944 relates to a process for the manufacture of trichloroethylene by reaction of tetrachloroethane with a nitrogen base. Diortho-tolyl guanidine is mentioned on page 1, right-hand column, line 32; this is the sole guanidine base in a list of different nitrogen bases. The examples teach the use of triethylamine and quinolone.
The use of certain amines or their salts in the dehydrochlorination reaction of chlorinated hydrocarbons has been previously disclosed.
U.S. Pat. No. 2,879,311 (THE DISTILLERS COMPANY LIMITED) Mar. 24, 1959 discloses a process for the dehydrochlorination of 1,2,3-trichlorobutane carried out in the presence of the hydrochloride or ammonium chloride salt of an amine having a pK in the range of 3.0 to 9.0. Exemplary amines suitable for the preparation of the salt are preferably tertiary amines such as: alpha- and gamma-picoline, 2,4,6-collidine, 5-ethyl-2-methyl-pyridine, tri-n-butylamine, pyridine, quinoline, N-ethylpyperidine, cyclohexylamine.
U.S. Pat. No. 2,989,570 (ETHYL CORPORATION) Jun. 20, 1961 discloses a process for the preparation of vinylidene chloride by dehydrochlorination of 1,1,2-trichloroethane in the presence of an amine, its hydrochloric or quaternary ammonium chloride salt. The amine is selected among those amines having a pKb (wherein Kb indicates the basic dissociation constant) of less than 7. Diphenylguanidine is listed among the suitable amines to be used in the process. The examples provided however do not indicate the existence of a catalytic process, rather of a stoichiometric reaction.
U.S. Pat. No. 5,210,344 (THE DOW CHEMICAL COMPANY) May 11, 1993 discloses a process for the preparation of vinylidene chloride by dehydrochlorination of 1,1,2-trichloroethane in the presence of a cyclic amine having a pKa greater than 11. Exemplary cyclic amines with a pKa greater than 11 are 2,2,6,6-tetramethylpiperidine, 1,2,2,6,6-pentamethylpiperidine, 2,2,4-trimethylpiperidine, 1,8-diazabicyclo-[5.4.0]-undec-7-ene, and 1,5-diazabicyclo-[4.3.0]-non-5-ene.
It has now been found that the use of certain guanidinium salts or their guanidine precursors provides sustainable catalytic processes for the dehydrochlorination of chlorinated hydrocarbons to produce unsaturated compounds in good yields. The use of said guanidinium salts, or their guanidine precursors, in the dehydrochlorination reaction of 1,1,2-trichloroethane is selective in the production of vinylidene chloride thereby reducing the amount of by-products. Hydrogen chloride generated in the process may be easily recovered in anhydrous form.